1. Field of the Invention
The present invention relates to an apparatus for detecting a fault in an exhaust gas recirculation (EGR in abbreviation) control system of an internal combustion engine for recirculating a part of exhaust gas of the engine to an intake pipe thereof.
2. Description of the Related Art
The EGR control systems of the type mentioned above are heretofore employed widely in the internal combustion engines for the purpose of reducing NO.sub.x content in the exhaust gas of the engine. As a typical one of such systems, there may be mentioned an EGR control system of exhaust gas pressure control type in which an exhaust gas pressure transducer is used. With this system, the amount of the exhaust gas to be recirculated is so controlled that the ratio of the exhaust gas contained in the intake air flow remains substantially constant.
In this conjuncture, an apparatus for detecting occurrence of abnormality or a fault in the EGR control system is also known, as is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 9937/1990 (JP-A-2-9937). According to this known technique, an exhaust gas recirculation (EGR) control valve is installed in an EGR passage through which an exhaust pipe of the engine is communicated to an intake pipe. The EGR control valve is temporarily operated (e.g. closed and opened) during deceleration of the engine, wherein it is detected whether or not a change in the pressure within the intake pipe brought about by the operation of the EGR control valve remains within a predetermined range. On the basis of the result of this decision, it is determined whether the EGR control system suffers from a fault.
The principle of the fault detecting apparatus described above is based on the fact that a change in the pressure within the intake pipe necessarily occur when the EGR control valve is actually operated. Starting from this fact, a negative pressure for actuating the EGR control valve is changed over for determining whether the change in the pressure within the intake pipe occurs due to the change-over of the negative pressure, to thereby diagnose the EGR control valve as well as peripheral devices thereof as to occurrence of the fault.
The fault detecting apparatus for the EGR control system described above has a problem in the application to the diagnosis of the EGR control system of the exhaust gas pressure control type in which an exhaust gas pressure transducer is employed and the amount of exhaust gas to be recirculated is so regulated that the exhaus gas content of the intake air flow remains at a substantially constant ratio, because a change in the pressure within the intake pipe is very small when the EGR control valve is operated during deceleration of the engine. Such being the circumstances, it becomes difficult or impossible in an extreme case to detect with reasonable accuracy whether the change of the pressure within the intake pipe lies within a predetermined range, particularly when the change of the intake pipe pressure is not sufficiently large as compared with the weight or margin imparted to a bit of a digital value obtained through analog-to-digital (A/D) conversion of the intake pipe pressure signal, i.e., when the change of the intake pipe pressure is less than the margin or threshold for a change of the last or lowest bit of the digital value. To cope with this problem (i.e., to detect a limited or fine change in the intake pipe pressure), the A/D converter for converting the signal indicative of the intake pipe pressure into a digital signal is required to have high resolution, which leads however to an expensive fault detecting apparatus.